1. Field of the Invention
This invention relates to a multi-plate type frictional engagement apparatus for changing over the transmission and non-transmission of motive power in the automatic transmission of a vehicle. More particularly, it relates to improvements in a separator plate used in the multi-plate type frictional engagement apparatus.
2. Related Background Art
Generally, a multi-plate type frictional engagement apparatus for changing over the transmission and non-transmission of motive power in the automatic transmission of a vehicle such as an automobile has a plurality of frictional plates and a plurality of separator plates disposed among them. During the non-transmission of the motive power, the frictional plates and the separator plates are spaced apart from each other with a predetermined clearance therebetween, but during the transmission of the motive power, the frictional plates and the separator plates assume a fastened state in which they are in contact with each other under pressure.
When shift is made from the fastened state to a non-fastened state and from the non-fastened state to the fastened state, and sometimes even in the fastened state, the frictional plates and the separator plates are in a relationship in which they fictionally slide relative to each other. Thus, during the operation of the frictional engagement apparatus, the frictional plates and the separator plates are exposed to a high temperature by frictional heat.
Heretofore, the separator plate has been made by working a thermally rolled material of ferrite crystal grain size number 12 or higher by cold rolling at a draft of the order to 50 to 60% and therefore, has become fibrous structure worked to such a degree that the grain diameter of the crystal thereof cannot be confirmed. Thereby, depending on a severe condition of use, with the frictional heat created by the frictional contact of the separator plate with the frictional plate as the main cause, the separator plate is recrystallized, quenched and tempered, and a minute protuberance or projection, i.e., a so-called "heat spot" is sometimes formed on the surface of the separator plate. If this protuberant heat spot is formed, when the frictional plate and the separator plate fictionally slide relative to each other, the heat spot attaches a frictional material attached to the frictional plate and may damage the frictional material. This has resulted in the problem that the service life of the apparatus becomes shorter.
Also, from the present day energy and environmental problems, the frictional engagement apparatus is required to be compact and light in weight and be high in durability, particularly durability against heat. From this point also, it is desired to supress the creation of the heat spot.